.\" $OpenBSD: BIO_f_md.3,v 1.15 2023/04/28 16:20:01 schwarze Exp $
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.Dd $Mdocdate: April 28 2023 $
.Dt BIO_F_MD 3
.Os
.Sh NAME
.Nm BIO_f_md ,
.Nm BIO_set_md ,
.Nm BIO_get_md ,
.Nm BIO_get_md_ctx ,
.Nm BIO_set_md_ctx
.Nd message digest BIO filter
.Sh SYNOPSIS
.In openssl/bio.h
.In openssl/evp.h
.Ft const BIO_METHOD *
.Fo BIO_f_md
.Fa void
.Fc
.Ft long
.Fo BIO_set_md
.Fa "BIO *b"
.Fa "EVP_MD *md"
.Fc
.Ft long
.Fo BIO_get_md
.Fa "BIO *b"
.Fa "EVP_MD **mdp"
.Fc
.Ft long
.Fo BIO_get_md_ctx
.Fa "BIO *b"
.Fa "EVP_MD_CTX **mdcp"
.Fc
.Ft long
.Fo BIO_set_md_ctx
.Fa "BIO *b"
.Fa "EVP_MD_CTX *mdc"
.Fc
.Sh DESCRIPTION
.Fn BIO_f_md
returns the message digest BIO method.
This is a filter BIO that digests any data passed through it.
It is a BIO wrapper for the digest routines
.Xr EVP_DigestInit 3 ,
.Xr EVP_DigestUpdate 3 ,
and
.Xr EVP_DigestFinal 3 .
.Pp
.Fn BIO_set_md
sets the message digest of
.Fa b
to
.Fa md
and initializes it using
.Xr EVP_DigestInit_ex 3 .
Calling this function is required before any data is passed through
.Fa b .
.Pp
.Fn BIO_get_md
places a pointer to the digest method of
.Fa b
into
.Pf * Fa mdp .
.Pp
Any data written or read through a digest BIO using
.Xr BIO_read 3
and
.Xr BIO_write 3
is digested.
.Pp
.Xr BIO_gets 3 ,
if its
.Sy size
parameter is large enough,
finishes the digest calculation and returns the digest value.
.Xr BIO_puts 3
is
not supported.
If an application needs to call
.Xr BIO_gets 3
or
.Xr BIO_puts 3
through a chain containing digest BIOs,
this can be done by prepending a buffering BIO.
.Pp
After the digest has been retrieved from a digest BIO, call
.Xr BIO_reset 3
to reinitialize it and any BIOs following it in its chain
before passing any more data through it.
If no subsequent BIOs require reinitialization,
.Fn BIO_set_md
can be used instead of
.Xr BIO_reset 3 .
.Pp
.Fn BIO_get_md_ctx
places a pointer to the digest context of
.Fa b
into
.Pf * Fa mdcp
and marks the BIO as initialized without actually initializing it.
Unless
.Fn BIO_set_md
was already called on
.Fa b ,
the caller becomes responsible for initializing the digest context with
.Xr EVP_DigestInit_ex 3 .
.Pp
The context returned by
.Fn BIO_get_md_ctx
can be used in calls to
.Xr EVP_DigestFinal 3
and also in the signature routines
.Xr EVP_SignFinal 3
and
.Xr EVP_VerifyFinal 3 .
.Pp
The context returned by
.Fn BIO_get_md_ctx
is an internal context structure.
Changes made to this context will affect the digest BIO itself, and
the context pointer will become invalid when the digest BIO is freed.
.Pp
.Fn BIO_set_md_ctx
replaces the digest context of
.Fa b
with
.Fa mdc .
Calling this function is usually not necessary
because creating a digest BIO with
.Xr BIO_new 3
automatically creates a digest context and stores it internally.
Before calling
.Fn BIO_set_md_ctx ,
the caller has to retrieve the old context using
.Fn BIO_get_md_ctx ,
and the caller also becomes responsible for calling
.Xr EVP_MD_CTX_free 3
on the old context.
Unless
.Fa mdc
is already initialized, the caller needs to initialize it after calling
.Fn BIO_set_md_ctx
using either
.Fn BIO_set_md
or
.Xr EVP_DigestInit 3 .
.Pp
When a chain containing a message digest BIO is copied with
.Xr BIO_dup_chain 3 ,
.Xr EVP_MD_CTX_copy_ex 3
is called internally to automatically copy the message digest context
from the existing BIO object to the new one,
and the init flag that can be retrieved with
.Xr BIO_get_init 3
is set to 1.
.Pp
.Xr BIO_ctrl 3
.Fa cmd
arguments correspond to macros as follows:
.Bl -column BIO_C_GET_MD_CTX "corresponding macro" -offset 3n
.It Fa cmd No constant  Ta corresponding macro
.It Dv BIO_C_GET_MD     Ta Fn BIO_get_md
.It Dv BIO_C_GET_MD_CTX Ta Fn BIO_get_md_ctx
.It Dv BIO_C_SET_MD     Ta Fn BIO_set_md
.It Dv BIO_C_SET_MD_CTX Ta Fn BIO_set_md_ctx
.It Dv BIO_CTRL_RESET   Ta Xr BIO_reset 3
.El
.Sh RETURN VALUES
.Fn BIO_f_md
returns the digest BIO method.
.Pp
When called on a message digest BIO object,
.Xr BIO_method_type 3
returns the constant
.Dv BIO_TYPE_MD
and
.Xr BIO_method_name 3
returns a pointer to the static string
.Qq message digest .
.Pp
.Fn BIO_set_md
returns 1 on success or 0 if
.Xr EVP_DigestInit_ex 3
fails.
.Pp
.Fn BIO_get_md
and
.Fn BIO_set_md_ctx
return 1 on success or 0 if
.Fa b
is not initialized.
.Pp
.Fn BIO_get_md_ctx
returns 1 on success or 0 on failure,
but the current implementation cannot actually fail.
.Sh EXAMPLES
The following example creates a BIO chain containing a SHA-1 and MD5
digest BIO and passes the string "Hello World" through it.
Error checking has been omitted for clarity.
.Bd -literal -offset 2n
BIO *bio, *mdtmp;
const char message[] = "Hello World";
bio = BIO_new(BIO_s_null());
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_sha1());
/*
 * For BIO_push() we want to append the sink BIO
 * and keep a note of the start of the chain.
 */
bio = BIO_push(mdtmp, bio);
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_md5());
bio = BIO_push(mdtmp, bio);
/* Note: mdtmp can now be discarded */
BIO_write(bio, message, strlen(message));
.Ed
.Pp
The next example digests data by reading through a chain instead:
.Bd -literal -offset 2n
BIO *bio, *mdtmp;
char buf[1024];
int rdlen;

bio = BIO_new_file(file, "rb");
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_sha1());
bio = BIO_push(mdtmp, bio);
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_md5());
bio = BIO_push(mdtmp, bio);
do {
	rdlen = BIO_read(bio, buf, sizeof(buf));
	/* Might want to do something with the data here */
} while (rdlen > 0);
.Ed
.Pp
This next example retrieves the message digests from a BIO chain
and outputs them.
This could be used with the examples above.
.Bd -literal -offset 2n
BIO *mdtmp;
unsigned char mdbuf[EVP_MAX_MD_SIZE];
int mdlen;
int i;

mdtmp = bio;	/* Assume bio has previously been set up */
do {
	EVP_MD *md;
	mdtmp = BIO_find_type(mdtmp, BIO_TYPE_MD);
	if (!mdtmp)
		break;
	BIO_get_md(mdtmp, &md);
	printf("%s digest", OBJ_nid2sn(EVP_MD_type(md)));
	mdlen = BIO_gets(mdtmp, mdbuf, EVP_MAX_MD_SIZE);
	for(i = 0; i < mdlen; i++)
		printf(":%02X", mdbuf[i]);
	printf("\en");
	mdtmp = BIO_next(mdtmp);
} while(mdtmp);
BIO_free_all(bio);
.Ed
.Sh SEE ALSO
.Xr BIO_new 3 ,
.Xr EVP_DigestInit 3
.Sh HISTORY
.Fn BIO_f_md ,
.Fn BIO_set_md ,
and
.Fn BIO_get_md
first appeared in SSLeay 0.6.0.
.Fn BIO_get_md_ctx
first appeared in SSLeay 0.8.1.
These functions have been available since
.Ox 2.4 .
.Pp
.Fn BIO_set_md_ctx
first appeared in OpenSSL 0.9.7e and has been available since
.Ox 3.8 .
.Pp
Before OpenSSL 1.0.0, the call to
.Fn BIO_get_md_ctx
would only work if the
.Vt BIO
had been initialized, for example by calling
.Fn BIO_set_md .
.Sh BUGS
The lack of support for
.Xr BIO_puts 3
and the non-standard behaviour of
.Xr BIO_gets 3
could be regarded as anomalous.
It could be argued that
.Xr BIO_gets 3
and
.Xr BIO_puts 3
should be passed to the next BIO in the chain and digest the data
passed through and that digests should be retrieved using a separate
.Xr BIO_ctrl 3
call.
